Petrochemical industry is an important supporting industry in national economy, and supplies a large quantity of chemical raw materials for various departments including industry, agriculture, communication and national defense, which is thus one of the industrial sectors taking correlative and leading action in national economy. Lower olefins are one of the most important basic raw materials constituting modern petrochemical industry.
For instance, propylene is mainly used for the production of polypropylene, cumene, oxo alcohol, acrylonitrile, propylene oxide, acrylic acid, isopropanol and etc., wherein polypropylene accounts for more than half of the demand for propylene in the world.
At present, 67% of propylene in the world is derived from by-products in the production of ethylene by steam cracking, 30% of which is derived from by-products in the production of gasoline and diesel oil by catalytic cracking unit (FCC) in refinery, and low amount of which (about 3%) is obtained from dehydrogenation of propane and metathesis reaction of ethylene-butylene. It is predicted that the demand of propylene in the future will be increased in a higher rate than the supply thereof. Considering the relatively higher rate of increase in term of demand of propylene, and the situation of “demand exceeds supply” presented in conventional production modes, it is necessary to recur to other various new techniques of increasing yield of propylene for the purpose of supplementing the demand of propylene.
In recent periods, under the influence of strong demand for polyolefins and alkyl aromatic compounds, the demand of lower olefins tends to be rapidly increased, while the conventional production modes of lower olefins cannot satisfy the rapidly increased demand of the market for lower olefins, thus it is necessary to recur to other various new techniques of increasing yield of lower olefins for the purpose of supplementing the demand of lower olefins.
On the other hand, there are quite a lot of raw materials of C4 or higher olefins in the world. Considering the influence of various factors including change of chemical product market and transportation cost, it is a preferable way to make use of these raw materials by subjecting them to deep processing on the spot. One hopeful process of which involves the conversion of C4 or higher olefins to lower olefins. The process not only can make use of raw materials of C4 or higher olefins being relatively surplus and having a lower accessory value, but also can obtain various lower olefins products having wide uses.
The reference document CN1490287A disclosed a process for production of ethylene and propylene by reacting a mixture containing C4 or C5 olefins in a fixed-bed reactor at a temperature of 350-500° C., a pressure of 0.6-1.0 MPa and a weight hourly space velocity (WHSV) of 1-10 hr−1. It was focused on the modification of various types of catalysts and the reaction results, and the reaction raw materials are mainly directed to C4 and C5 olefins but not to C5 or higher olefins. Meanwhile, a single reactor configuration was used therein, thus it was impossible to assure that the catalyst kept a desirable stability and to obtain a desirable yield of the target products.
The reference document CN1274342A (counterpart to U.S. Pat. No. 6,307,117B1) disclosed a process for producing ethylene and propylene by catalytic conversion from a linear hydrocarbon feedstock containing 20% or more of at least one C4-C12 olefins, wherein zeolite in a zeolite-containing catalyst used therein satisfied the following requirements: said zeolite contained substantially no proton, said zeolite had a SiO2/Al2O3 molar ratio of from 200 to 5,000, and said zeolite contained at least one metal selected from the group consisting of metals belonging to Group IB of the Periodic Table, and said zeolite was an intermediate pore size zeolite, and the preferred zeolite belonged to the ZSM-5 zeolite family. The reaction was carried at a temperature of 400-700° C., a pressure of 0.1-10 atm and a WHSV of 1-1,000 hr−1. However, a single reactor configuration was similarly used therein, thereby resulting in relatively lower yields of ethylene and propylene with the highest yield of propylene being only 25.19%.
The reference document WO 00/26163 to Equistar Chemicals, L.P. disclosed a process for making propylene and ethylene from a feedstock containing at least 60 wt. % C4 and/or C5 olefins with a zeolite catalyst having an intermediate pore size. Zeolites useful in the invention included: zeolites having one-dimensional channel such as ZSM-23 and AlPO4-11 which had a pore diameter greater than 3.5 Å and a pore size index within the range of 14 to 28; and zeolites having interconnecting channels such as ZSM-57 and AlPO4-18 which included a primary channel that had a pore diameter greater than 3.5 Å and a pore size index within the range of 14 to 28, and a secondary channel that had a pore size index less than 20. The catalyst could be Na-type, H-type and etc., in which trace amounts of an oxidizing metal such as Pd or Pt could be added to promote coke removal during catalyst regeneration. The process was used generally with a fixed-bed reactor system, and the reaction was preformed at a temperature of 200-750° C., a pressure of 0.05-1 MPa and a WHSV of 0.5-1,000 hr−1. On the one side, the reference did not disclose concrete preparation method of the catalyst and reaction data. Meanwhile, a single reactor configuration was also used therein, which determined the results including non-ideal yields of ethylene and propylene and poor stability of the catalyst.
The reference document EP0109059A1 put forward a process for conversion of C4-C12 olefins to propylene. The catalyst used therein was ZSM-5 zeolite having a SiO2/Al2O3 molar ratio equal to or lower than 300. The process was carried out at a temperature of 400-600° C., and a space velocity of olefins of higher than 50 hr−1. The influences of various formulated raw materials, reaction temperature and space velocity on catalytic cracking reaction were investigated in detail therein. However, it was not mentioned in this invention to use combined reactor configurations, thus the catalyst could not effectuate its advantage of optimum catalytic conversion, and the target product did not have satisfactory selectivity and stability.
The reference document U.S. Pat. No. 5,981,818 put forward a process for conversion of C4-C7 olefins to C3 and C4 olefins. The catalyst used therein was pentasil type zeolite catalyst having a SiO2/Al2O3 molar ratio of 10-200, a BET specific surface area of 300-600 m2/g, and a particle size of 0.1-0.9 μm. The reaction temperature was 380-500° C. One feature of this invention resided in blending a certain proportion of vapor into the feedstock, wherein the ratio of H2O/HC was 0.5-3 (by weight). It was said that the addition of vapor could alleviate carbon deposition on the catalyst to thereby increase stability of the catalyst. Likewise, it was not considered in this invention to use combined reactor configuration, which similarly could not effectuate the advantage of optimum activity and stability for the catalyst.